The detection and analysis of vibration in machines having large rotating components has long been known and utilized as a technique to detect incipient failures. Even techniques of limited reliability were deemed desirable because of the catastrophic consequences of failures, not only in terms of the destructive potential of a failed rotating component, but also because of the economic losses resulting during the time required to repair or replace such components or entire machines.
The problem of incipient failure is a more serious one when the machine with the rotating component is an aircraft jet engine. The modern engine includes several concentrically mounted shafts, each rotating at a slightly different frequency. Each of the rotating shafts or spools can include a compressor-turbine assembly which in turn includes fan and turbine blades. In the intake stages the blades are used to compress incoming air, and in the output stages the blades drive the compressor.
Machinery having massive rotating components, such as jet aircraft engines but also including large motor generators, turbines and the like, may experience bearing failures or may, through problems of wear or accident, become unbalanced and impose unacceptable loads upon the bearing and the shaft housings.
The problem, of course, is of greatest gravity when a passenger-carrying jet airliner is involved. The large fans and turbines, which are integral parts of the jet engine, can, upon catastrophic failure, penetrate the aircraft hull and cause substantial injury to the cabin and occupants, as well as impair the air worthiness of the aircraft.
For some time, techniques have been available to monitor the vibrations of rotating machinery and to signal dangerously high vibration amplitudes, or, at least, signal large incremental changes over the otherwise normal patterns of vibration. Such techniques are also available to assist in the balancing of the rotating components to keep vibrations at acceptably low levels. Limiting vibration has long been deemed a factor in prolonging the life of the bearings.
While such techniques are applicable to aircraft, the environment of the modern jet engine tends to create a high "noise" level due to the sympathetic vibrations of component parts of the aircraft. When operating, the jet engine generates a broad spectrum of frequencies including harmonics which usually excite most if not all resonances. Accordingly, it is difficult to monitor the vibrations that are directly related to the main rotating components of an engine in the presence of all of the other components of "noise."
Prior art techniques have utilized filters in an attempt to isolate the vibrations attributable to the engine components, and these filtered signals are then processed to provide a quantitative display that a trained observer could interpret. The observer, noting the amplitude over a period of time, can then judge if a malfunction is threatened or if one exists.
In the prior patent to Cochard, U.S. Pat. No. 4,213,114 of July 15, 1980, a system was disclosed utilizing collocated transducers which were alternately sampled. A broad-band channel is used which includes a broad-band filter whose output is integrated, rectified and, if selected, can be displayed. The integrater output is also applied to two or more narrow band channels corresponding to the coaxial shafts which have different frequencies of rotation.
A tachometer is associated with each shaft and is used to control phase-locked loop frequency multipliers whose outputs are applied to monostable circuits which control conventional analog tracking filters, the output signals of which depend only on the amplitude of the basic frequency of the input signal from the integrater.
It has been found that analog circuits, in general, must be designed for specific applications, and, further, tend to be susceptible to noise and electrical disturbances which could adversely affect the integrity of the output signals. Further, the frequencies of interest, which range from 20 Hz to 200 Hz, are not easily accomodated in analog circuits.
A novel system based upon a digital computer has been created and has been disclosed in the copending application of David Ray and Nikul Kapadia, Ser. No. 344,561, filed Feb. 1, 1982, and assigned to the assignee of the present invention. The analog output of a transducer, such as an accelerometer was converted into digital signals. Tachometer signals were provided as a pulse train whose frequency was related to the frequency of the rotating component of interest. Standard digital components, including counters and memory devices, were utilized to generate a sampling pulse train whose frequency is a predetermined, integral fraction of the frequency of the rotating component, so that a suitable number of samples of the accelerometer output can be digitized to represent adequately the quantities sensed by the transducer.
A nonrecursive digital filter was created utilizing a memory in which coefficients are stored. Each digitized sample was processed through the digital filter to create a digitized output. The digital computer then converted the filter output to an RMS value which was converted in a digital to analog converter. The resultant analog signal was applied to drive a meter display.
Because a general purpose digital computer was employed in conjunction with memory, the apparatus could, through programming, be adapted to perform other functions. For example, vibrations at virtually any frequency of interest could be detected and displayed.
A problem that had not been dealt with heretofore involved the reliability of the several vibration transducers and the question of selecting from among the transducers providing signals of differing magnitude. Prior systems, such as Cochard, utilized a pair of collocated transducers on the assumption that both would not fail in identical fashion. The prior systems recognized that output signals of differing magnitudes can be recognized as a system "fault" and might give rise to an alarm.
The important question was, which of the transducers is providing the correct reading? The transducer providing the signal of the dangerous vibration condition might be defective and another transducer, signalling vibration within safe limits might be correct and should be believed. The prior art solution was to signal a "fault" if either transducer signalled an out-of-limit vibration or if their signals differed by more than a permitted amount.
It has been discovered that it is possible, using the information that is available in the system, to derive a signal representing the background noise component from each of the transducers. The transducer signalling the least background noise can then be deemed most credible. Alternatively, one can normalize the noise components by the total signal values for each transducer and select, as most credible, the transducer with the lowest normalized noise component.
Yet another alternative method would be to average the data signals and use the averaged value to normalize the noise components, and selecting the transducer having the lowest normalized noise value. All of these methods can be implemented as simple computations considering the data that is available to a data processor in digital form, using the system of the copending Ray-Kapadia application.